Such an advancing unit is known from DE 10 2011 001 712 A1. It is a component of a current collector system, which is fastened to a supporting leg of a crane and can be moved with it along a line on which at least one current track is located. A current collector unit of the current collector system can produce an electric contact with the current track, wherein, for the purpose, the current collector unit can be moved to an introduction device of the path, via the advancing unit which can be displaced in the horizontal direction. The current collector unit can be moved in a vertical direction, supported in a guide on the horizontal advancing unit, wherein current collector contacts of the current collector unit can be moved to the current track in a vertical direction by means of the introduction device, so as to produce an electric contact with it.
U.S. Pat. No. 2,190,213 A1 discloses a flexible cover for a guide track on a tool machine, wherein the cover has the form of a band, which is rolled off from a movable carriage from a spring drum that is placed on the end of the guide track when the carriage is moved away from the spring drum, and is pulled into the spring drum when the carriage is moved toward the spring drum.
DE 940 798 discloses a protective device for tool machines to cover openings that are needed in order to connect a displaceable part of the tool machine with a drive means. An elastic band, which, without the effect of external forces, assumes a stretched or nearly stretched position, is used as a cover band, wherein the axle intended for the winding up of the band is stationary and the band is placed around this axle with a number of windings, in the form of a spiral and under pretension.
Current collector systems of the type mentioned in the beginning are normally used in the open air and are therefore exposed to unfavorable environmental and weather influences. Among these are moisture and soiling due to the deposition of dust or sand and, in the winter, the deposition of snow and the formation of ice by freezing moisture. This can lead to increased friction, corrosion, and, in the worst of cases, to a blocking of the movability of an advancing unit in the area of the running surfaces of guides. A horizontal advancing unit is hereby particularly endangered, because its running surfaces lie approximately horizontal and thus are particularly susceptible for depositions of the previously mentioned type.